1. Field of the Invention
The present invention relates to an injection unit of an injection molding machine.
2. Description of Related Art
An injection unit of an injection molding machine has a front plate that is fitted with a heating cylinder and is fixed to a base of the injection unit and a rear plate connected to the front plate at a distance from each other with a tie bar or the like. Disposed between the front plate and the rear plate is a pusher plate to which an injection screw is fitted to be rotatable and axially unmovable, and disposed between the pusher plate and the rear plate is a drive mechanism for moving the pusher plate backward and forward. The pusher plate is guided by the tie bar or the like to move backward and forward, and the injection screw is then moved forward to perform injection.
The pusher plate is supported and guided by the tie bar or the like to be displaced, and the weight of the pusher plate is applied to the tie bar or the like, which results in the flexure of the tie bar or the like. The flexure of the tie bar not only deteriorates the slidability of the pusher plate but also causes the displacement of the central axis of the pusher plate. Moreover, extra load is applied to the drive mechanism, such as a ball screw, for driving the pusher plate, and shortens the life duration of the mechanism. Under the circumstances, an injection unit in which a pusher plate is supported and guided by a linear guide fitted to the base of the injection unit has been well known (see JP 2001-269974A). JP 2001-269974A further teaches providing a pusher plate height-adjusting mechanism using a wedge-shaped member.
There is another well-known invention in which no tie bar is disposed between the front plate and the rear plate, but a sidewall for connecting the front plate and the rear plate is provided. A spline shaft portion of an injection screw is borne by a bearing that constitutes a body frame together with the front and rear plates. There is also disposed a linear guide in the bottom face of the body frame, and a shaft-connecting portion where the linear motion member of a linear motor and the spline shaft of the injection screw are connected to each other is guided by the linear guide (see JP 2004-1279A).
In an injection unit, injection pressure at the time of injection is transmitted through the injection screw to the pusher plate and then transmitted to the rear plate to which the drive mechanism for driving the pusher plate is fixed. The injection force transmitted to the rear plate is applied to connecting members, such as a sidewall and a tie bar having relatively low rigidity, which connect the front plate and the rear plate, and deforms the connecting member to absorb the injection pressure. The deformation of the connecting members causes the displacement of the central axis of the injection screw. If the connecting members that receive the injection force have a configuration symmetric with respect to the screw shaft, the connecting members are equally stretched in the axial direction of the screw shaft, so that the superposition of the linear guide upon the connecting members does not cause the displacement of the central axis of the screw.
If the front and rear plates are connected by a floor in addition to the sidewall, however, the floor also receives the injection force in an injecting process and acts in the same manner as the sidewall.
FIG. 7 is a view for explaining deformation of connecting members and the like, attributable to an injection force at the time of injection. In FIG. 7, a front plate 1 and a rear plate 2 are connected to each other by sidewalls 3a and a base member 3b which constitute connecting members 3. A heating barrel 11 is fixed to the front plate 1, and an injection screw 12 is inserted in the heating barrel 11. The injection screw 12 is fitted to a pusher plate 5 to be rotatable and axially unmovable, and is driven to rotate by a motor for screw rotation or the like, not shown, through a pulley 15. A ball nut 7 is attached to the pusher plate 5 with a load cell 9 disposed therebetween. A ball screw 8 is screwed in the ball nut 7, and the ball screw 8 is supported by the rear plate 2 to be rotatable and axially unmovable. The ball screw 8 is driven to rotate by a motor for injection, not shown, through a pulley 14 provided to the end thereof, and causes the pusher plate 5 to move forward (from right to left in the drawing), to thereby perform injection.
Injection pressure (injection force) created by the injection is transmitted to the screw 12, the pusher plate 5, the ball nut 7, the ball screw 8, and the rear plate 2, and eventually deforms the connecting members 3 including the sidewalls 3a and the base member 3b which have relatively low rigidity. If arranged symmetrically with respect to a screw shaft, the connecting members 3 are equally stretched in the axial direction of the screw shaft. As illustrated in FIG. 7, however, there is a space in a plane (upper side of the injection unit) on the side that is axisymmetrical about the screw shaft to the base member 3b that functions in the same manner as the sidewall 3a, so that the rigidity thereof is lower than the other three planes, and the upper side of the injection unit is deformed to splay.
For that reason, as disclosed in JP 2001-269974A and JP 2004-1279A, if a supporting member for mounting the linear guide is fixed onto the floor, rectilinearity of axial displacement of a member corresponding to the pusher plate is maimed, and the displacement of the central axis of the screw occurs in relation to the heating barrel fixed to the front plate, which raises the fear that the inner face of the heating barrel and the screw are brought into contact with each other.
In case that the pusher plate is guided by the tie bar serving as a connecting member as described in JP 2001-269974A, there generates the problem that the tie bar itself is bent and deformed, and rectilinearity is lost, which deteriorates the slidability of the guide.
Typically in an injection molding machine, the exchange of a mold and a heating barrel is carried out by the end user. It is required in each case to align the mold with the axis height of a nozzle fitted to the end of the heating barrel. However, the adjustment of the axis height of the nozzle is a difficult work for the end user, and an unsuccessful work causes the displacement of the central axis of the screw in relation to the heating barrel.